The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
Spark-ignition (SI) engines can be configured to operate in a homogeneous-charge compression-ignition (HCCI) combustion mode, also referred to interchangeably as controlled auto-ignition (HCCI) combustion, under predetermined speed/load operating conditions. The controlled auto-ignition (HCCI) combustion includes a distributed, flameless, auto-ignition combustion process that is controlled by oxidation chemistry.
Controlled auto-ignition (HCCI) combustion is a distributed kinetically-controlled combustion process with the engine operating at a dilute air/fuel mixture, i.e., lean of an air/fuel stoichiometric point, with relatively low peak combustion temperatures, resulting in low nitrous oxides (NOx) emissions. The homogeneous air/fuel mixture minimizes occurrences of rich zones that form smoke and particulate emissions.
In an HCCI engine, phasing of auto-ignited combustion strongly depends on the cylinder charge temperature, composition, and cylinder pressure, at intake valve closing. Hence, the control inputs to the engine, such as amount of fuel, fuel injection timing, spark timing, EGR valve open position, and intake and exhaust valve profiles, must be carefully coordinated to ensure that those key cylinder variables are within a range where the auto-ignited combustion can be robustly achieved. Among those inputs, the amount of EGR available to the engine is controlled by the open position of the EGR valve and may be considered in terms of EGR mass flow rate. Control authority of the EGR valve relates to an ability to adjust the EGR mass flow rate by adjusting open position of the EGR valve. One having ordinary skill in the art understands that an EGR valve may have an open position that can be controlled between 0% to 100%, whereas the EGR mass flow rate can be adjusted for EGR valve open positions, e.g., between 0% to 30%, depending upon engine operating conditions. Increasing the EGR valve open position above a control authority threshold does not result in additional EGR mass flow rate at the engine operating conditions because the effective area of EGR valve does not increase much once EGR valve opens more than 30%, as in the present example. The control authority of the EGR valve can be determined by an engine control module based on various feedback parameters such as intake manifold pressure, absolute pressure, combustion phasing, temperature, and other engine parameters.
The EGR mass flow rate available to the engine is limited by an amount related to the pressure within the intake manifold and the EGR valve opening, e.g., if the intake manifold pressure is high, the maximum EGR mass flow rate may be limited before the EGR valve reaches a fully opened position. This limitation restricts the useable engine range as a significant amount of EGR is necessary at high load and high speed to slow down cylinder pressure rise thereby reducing combustion noise and achieving a higher maximum fuel rate.